1. Field of the Invention
Polymer dispersions on an aqueous basis are characterized by the presence of dispersely distributed polymer particles in an aqueous dispersion fluid. For stabilizing a system of this type in which discrete polymer particles, for thermodynamic reasons, tend to coagulate, as a rule, surface active substances are used. The capability of using starch and the derivatives thereof as surface active substances of this type for generating aqueous polymer dispersions has already repeatedly been suggested.
2. Description of the Related Art
EP 0 276 770 is concerned with a sizing agent in the form of an aqueous polymer dispersion wherein a monomer mix of 20-65% acrylonitrile, 80-35% acrylic acid ester and 0-10% of other ethylenically unsaturated monomers in the aqueous phase is copolymerized in one step, using modified starch of a reduced viscosity of 0.04 to 0.12 dl/g.
EP 0 536 597 A1 describes aqueous polymer dispersions obtainable by radical polymerisation of unsaturated monomers containing at least one reduced starch product obtainable by hydrolysis in the aqueous phase and having an average molecular weight of between 2.500 and 25.000. In none of the aforementioned cases, the way of carrying out the radical polymerisation is suitable for preparing an amphoteric copolymer. The afore-mentioned patent applications do not describe any charge characteristics actually required for these systems in order to judge their suitability for special applications, such as sizing and coating.
EP 0 479 245 A2 describe polymers usable as adsorbents for aqueous media, with the polymers being formed by copolymerisation of an ampholytic ion-pair and at least one additional monomer, wherein the said second monomer can be acrylamide, methacrylamide or acrylonitrile. Moreover, the said polymers also contain so called cross-linking agents carrying, in turn, at least 2 olefinic functional groups, with each of these groups being suitable for cross-linking. The polymer, in addition, contains polysaccharides, polypropylene or polyethylene forming in conjunction with the afore-mentioned comonomers a graft polymer. The resultant polymer exclusively contains water-soluble monomers and is formed in a one-step reaction. U.S. Pat. No. 4,552,940 discloses a starch-containing graft polymer generated in the form of an aqueous solution. It was the object of that invention to adjust the viscosity of the starch solution by the addition of styrene. The so adjusted starch solutions can then be employed as adhesives or coating agents. The problem underlying the present invention resides in providing amphoteric aqueous polymer dispersions which with the aid of a suitable process using starch and/or the derivatives thereof, can be generated free of coagulate. Polymer dispersions of this type, with a uniform particle size distribution, should be of a high stability and, through their surface charge, should be compatible both with anionic and cationic systems, and safeguard the use thereof over a wide pH-range.